1. Field of the Invention
The present invention relates to an apparatus for detecting the oxygen gas concentration in a sample gas. More particularly, the present invention relates to an apparatus attached to an exhaust tube of an internal combustion engine, which is suitable for detection of the oxygen gas concentration in the exhaust gas, which has a close relation to the air-fuel ratio of an air-fuel mixture supplied to the engine.
The apparatus may provide a feedback signal or the like for the feedback control of the air-fuel ratio, a fuel injection timing or an engine idle speed etc.
2. Description of the Related Art
As means for detecting the oxygen gas concentration in an exhaust gas of an internal combustion engine (hereinafter referred to as "oxygen sensor"), an apparatus disclosed in Japanese Patent Application Laid-Open Specification No. 203828/84 is known, and the main part for detecting the oxygen gas concentration in the oxygen sensor is known, for example, from Japanese Patent Application Laid-Open Specification No. 204365/83 or 16258/88.
Namely, the main part of the oxygen sensor has a plate type ceramic substrate composed mainly of zirconium oxide (ZrO.sub.2), and parts of the inner and outer surfaces of the ceramic substrate are coated with a platinum (Pt) paste and the ceramic substrate is then calcined to form a pair of electrodes for taking out an electromotive force. Furthermore, platinum is vacuum-deposited on the outer surface of the ceramic substrate to form an oxidation catalyst layer for oxidizing unburnt components in the exhaust gas, such as CO and HC. Then, a metal oxide such as magnesium spinel is flame-sprayed on the oxidation catalyst layer to form a protecting layer for protecting the oxidation catalyst layer.
In this structure, the air is introduced as the stable reference gas onto the inner surface of the ceramic substrate, and the outer side of the ceramic substrate is exposed to an exhaust gas passage of the engine and is contacted with the exhaust gas of the engine. A voltage corresponding to the ratio of the oxygen gas concentration in air contacted with the inner surface of the ceramic substrate to the oxygen gas concentration in the exhaust gas contacted with the outer surface of the ceramic substrate is generated between the pair of electrodes, and the oxygen gas concentration in the exhaust gas is detected based on this voltage.
It is considered that the electromotive force effect is generated between the electrodes on the inner and outer surfaces of the ceramic substrate according to the following mechanism.
If calcia (CaO) or yttria (Y.sub.2 O.sub.3) is added to zirconia (ZrO.sub.2) known as a main component of a ceramic and the mixture is heated, calcia or yttria is included in the crystal and a lattice defect of the oxygen ion is formed, whereby zirconia is formed into a pure oxygen ion conductor in which the oxygen ion moves though either the electron or the hole hardly moves. If the oxygen partial pressure on one wall of densified zirconia is made different from the oxygen partial pressure on the other wall, it is only the oxygen ion O.sup.2- that can move, and as the result, an electromotive force is generated.
Incidentally, the oxidation catalyst layer of platinum promotes oxidation reactions of CO+1/2O.sub.2 .fwdarw.CO.sub.2 and HC+O.sub.2 .fwdarw.H.sub.2 O+CO.sub.2 between oxygen and carbon monoxide CO or hydrocarbon HC, and when combustion is carried out with a richer air-fuel mixture, CO or HC is conveniently reacted with low-concentration oxygen left in the air-fuel mixture to reduce the oxygen concentration almost to zero, whereby the oxygen concentration ratio between the inside and outside of the ceramic substrate is increased and a large electromotive force is generated. On the other hand, when combustion is carried out with a leaner air-fuel mixture since high-concentration oxygen and low-concentration CO and HC are present in the exhaust gas, even if oxygen reacts with CO and HC, O.sub.2 still remains in a considerable amount, and the oxygen concentration ratio between the inside and outside of the ceramic substrate is low and no substantial voltage is produced.
Since the value of the electromotive force output from the oxygen sensor abruptly changes in the vicinity of the stoichiometric air-fuel ratio as pointed out above, by utilizing this phenomenon, it is judged whether or not the air-fuel ratio in an air-fuel mixture sucked in the engine is the stoichiometric air-fuel ratio. If the air-fuel ratio is richer than the stoichiometric air-fuel ratio, the amount of the fuel to be supplied into the engine is decreased or the amount of the intake air is increased, and if the air-fuel mixture is leaner than the stoichiometric air-fuel ratio, the amount of the fuel is increased or the amount of the intake air is decreased. Thus, feedback control of the air-fuel ratio is performed.
However, in the above-mentioned conventional oxygen sensor, the oxidation catalyst layer has no substantial effect of reducing nitrogen oxides NO.sub.x, and therefore, the oxygen concentration in the exhaust gas is detected irrespectively of the concentration of nitrogen oxides NO.sub.2.
Incidentally, nitrogen oxides NO.sub.x are formed by bonding of nitrogen N.sub.2 in the air to oxygen in a high temperature atmosphere.
Namely, oxygen in NO.sub.x should be detected as oxygen concentration, which has not made any contribution to combustion, for detection of the air-fuel ratio, but this oxygen is not detected by the conventional oxygen sensor.
Accordingly, the detection value of the oxygen sensor is changed by the amount corresponding to the amount of oxygen which has reacted with nitrogen gas N.sub.2 to form NO.sub.x, and in the air-fuel ratio region where the detection value of the oxygen sensor is inverted, the apparent air-fuel ratio can not correspond to the actual air-fuel ratio and can not be stable by the change of amount of the NO.sub.x concentration even when the actual air-fuel ratio is constant.
Therefore, if feedback control of the air-fuel ratio is performed according to the detection result based on the air-fuel ratio in the inversion region of the oxygen sensor as the reference, the air-fuel ratio is erroneously controlled to an unpreferable level and there is a risk that oxidation reaction of nitrogen gas is advanced and nitrogen oxides NO.sub.x in the exhaust gas are excessive.
In general, a ternary catalyst for purging the exhaust gas, which is disposed in the exhaust gas passage in the engine, can simultaneously convert CO, HC and NO.sub.x efficiently when the air-fuel ratio is close to the stoichiometric air-fuel ratio, but if the air-fuel ratio is not controlled to a desired level i.e. the stoichiometric air-fuel ratio, the conversion of NO.sub.x is abruptly reduced and the amount of NO.sub.x discharged to the air present downstream of the ternary catalyst passage is drastically increased.
According to the conventional technique, so-called exhaust gas recycle (EGR) control for reducing nitrogen oxides NO.sub.x operates by recycling a part of the exhaust gas of the engine into the intake air and thus lowering the combustion temperature. However, the structure of this EGR control system is complicated because an EGR passage should be laid out and an EGR control valve or the like should be disposed in this passage, and this results in increase of the cost. Moreover, the combustion efficiency is reduced by introduction of the exhaust gas and the fuel expense is greatly increased.
Accordingly, if feedback control of the air-fuel ratio of the internal combustion engine is performed by the conventional inaccurate oxygen sensor, excessive discharge of nitrogen oxides NO.sub.x cannot be avoided, and in order to prevent nitrogen oxides NO.sub.x from being discharged to the outside, the EGR control system should be disposed in the internal combustion engine, which inevitably resides in the above-mentioned disadvantages.
In order to solve the above-described conventional problems, an oxygen sensor in which a nitrogen oxide-reducing catalyst layer for promoting the reduction reaction of nitrogen oxides has been provided as U.S. patent application Ser. No. 117,507 (U.S. Pat. No. 4,773,376) by the present applicant. Output characteristics of the proposed oxygen sensor keep stable even if the amount of the nitrogen oxides in the exhaust gas changes in case where the air-fuel ratio of the intake mixture maintains at a constant level.
However, since the oxygen sensor with the nitrogen oxides layers is constructed by using a test tube shaped ceramic tube having the top end closed and since the nitrogen oxide-reducing catalyst layer is formed on an outer curved surface of the ceramic tube, the thickness of the nitrogen oxide-reducing catalyst layer becomes inevitably uneven. Accordingly, the reaction is dense in the top end portion of the ceramic tube while the reaction is sparse on the side of the base end, and because of this difference of the reaction density, the degree of deterioration is made locally different in the oxygen sensor.
Further, it may be required for the proposed oxygen sensor with the nitrogen oxides layer to be heated so that the activation temperature thereof can be attained without a long time consumption when the exhaust gas is low and selection of kinds of reducing catalysts or the carrier for supporting the catalyst is not limited.
In case of the tube type oxygen sensing element, a rod-shaped heater may be placed in an inner cavity of a ceramic tube having the top end closed and an element is heated by this heater. However, in the structure where the heater as a separate body is inserted in the inner cavity of the ceramic tube, increase of the cost by the disposition of the heater cannot be avoided, and furthermore, since the heater is not directly contacted with the element (ceramic tube) but the element is heated through an air layer, the efficiency of the heating by the heater is low and a long time is required for heating the element at a temperature where the characteristics become stable.